This invention relates to the field of digital transmission systems, more particularly to the transmission of audio program signals over T carrier.
The Gaunt et al. and Sharper applications mentioned above describe apparatus that is very useful for preparing an audio program signal for transmission over a T1 digital transmission line. Such a signal may carry, for example, a symphony concert from a concert hall to a radio transmitter for broadcast over FM stereo radio. According to the Gaunt-Giammusso application, the music signal could be sampled at a 32 kHz rate, the samples converted to a 12-bit compressed PCM code word and four 12-bit words inserted into six 8-bit time slots of a T1 line. To make efficient use of all twelve bits and insure timing recovery at the T1 repeaters, Gaunt et al. teaches a compressing characteristic of seven chords each represented in the 12-bit word by three centrally located chord bits comprising at least one "1". Such program channels may be carried on the same T1 line with ordinary voice signals.
T1 carrier, being designed primarily for telephone service, is a bidirectional system using a separate pair of wires for each direction. For maintenance purposes, when a fault occurs in one direction of transmission, a message to that effect is sent in the opposite direction. With most channel banks that terminate T1 lines, the form of that message, known as a "yellow" alarm, is a pre-emption of the second bit in each 8-bit time slot. Audio program signals, on the other hand, are generally unidirectional; a fault occurring in a T1 line in the direction toward the concert hall or studio would not affect the program signal. The "yellow" alarm, however, by pre-empting important information bits of each word, would have a devastating effect on the program signal.
An object of this invention is a system for preparing program signals for transmission over T carrier which renders a yellow alarm unnoticeable in the recovered program signal.